Artificial muscles seek to replicate or mimic the versatility and capability of natural skeletal muscles in exerting a mechanical force, but using electrical energy. Accordingly, an artificial muscle forms a useful electrical-mechanical transducer or actuator, exerting a mechanical force through contraction and/or expansion.
Skeletal muscle is an amazingly versatile linear actuator. Traditional actuation technologies such as piezoelectrics, electromagnetics, and shape memory alloys may be capable of out-performing skeletal muscle in specific areas (e.g. speed, pressure, or energy density) but none are capable of operating effectively in as wide a range of conditions as muscle. By being highly compliant when not activated and only recruiting individual muscle units as they are required, skeletal muscle has great scope for optimizing efficiency for a wide range of loads and speeds.
Mimicking the well-rounded performance and characteristics of skeletal muscle with an artificial actuator has not been possible with traditional technologies. Electromagnetic motors for instance are heavy and rigid, and often must be coupled with a gearbox in order to achieve a useful output, with each additional component or moving part adding its own inherent losses and complexity to the system. Piezoelectrics are capable of high active speeds and pressures, but unlike muscle, they are extremely brittle and have very small output strains. Shape memory alloys can produce high pressures and moderate strains, but are slow and susceptible to fatigue loading. Dielectric Elastomer Actuators (DEAs), however, present a very promising alternative to these traditional technologies. DEA performance in terms of strain, speed, and energy density compare very favourably with those of skeletal muscle, and importantly their low material density, compliant nature, and silent operation capture many of the desirable physical properties of muscle.
Referring to FIGS. 1(a) and 1(b), a DEA generally referenced 10 comprises a dielectric elastomer membrane 11 provided between compliant electrodes 12. The dielectric elastomer membrane 11 is compressed by electrostatic pressure when a high voltage is applied across the electrodes 12 in the manner of a capacitor, causing planar expansion of the polymer from the uncompressed or contracted state as shown in FIG. 1(a), to the compressed or expanded state illustrated in FIG. 1(b).
Natural muscle, however, is much more than just an actuator, as it provides position feedback to the brain. Specialized cells within muscle tissue provide feedback to the body's central nervous system and this information is crucial to the coordination of muscle groups necessary for maintaining balance and posture. In an extreme case, that feedback may include a pain signal when there is a danger of overexertion causing damage to the muscle or other parts of the body. Automatic reflex actions in response to this feedback can even occur without conscious thought, particularly in an attempt to prevent harm e.g. recoiling from a sharp object. Skeletal muscle is a key component in the distributed control system that is the human body.
Because a DEA is constructed from a material which is resistant to compression, it is possible to relate a change in capacitance to changes in the physical geometry of the DEA. “An adaptive control method for dielectric elastomer devices” (Todd A. Gisby, Emilio P. Calius, Shane Xie, and lain A. Anderson, Proc. SPIE, 2008), the contents of which are incorporated herein by way of reference, discloses the use of self-sensing based upon the capacitance between electrodes to determine the state of a DEA, thereby providing some feedback. Similar methods are disclosed by “Control system design for a dielectric elastomer actuator: The sensory subsystem” (Toth, L. A. and A. A. Goldenberg, Proceedings of SPIE, 2002) and “A self-sensing dielectric elastomer actuator” (Jung, K., K. J. Kim, and H. R. Choi, Sensors and Actuators A: Physical, 2008).
However, existing self-sensing methods are accurate only under certain circumstances, such as when the DEA is stationary (i.e. not subject to any perturbations caused by external forces), the leakage current is negligible, and/or for low actuation speeds, or are based upon assumptions which may not always hold true. In addition, the methods of the prior art may not be suitable for practical implementation in a system designed for portable use. Accordingly, there is currently no satisfactory method for accurately determining the capacitance or state of a dielectric elastomer actuator, or providing feedback of movement of an artificial muscle using self-sensing.